1. Field of the Invention
The invention relates to novel crosslinkable, photoactive polymers and their use as orientation layers for liquid crystals and for the production of unstructured or structured optical elements and multilayer systems.
The orientation layer is particularly important in (electro-optical) liquid crystal devices. It serves for ensuring uniform and trouble-free orientation of the longitudinal axes of the molecules.
Uniaxially rubbed polymer orientation layers, such as, for example, polyimide, are usually used for orienting liquid crystal molecules in liquid crystal displays (LCDs). The direction of rubbing determines the orientation direction in this process. However, rubbing entails some serious disadvantages which may strongly influence the optical quality of liquid crystal displays. Thus, rubbing produces dust which may lead to optical defects in the display. At the same time, the polymer layer is electrostatically charged, which, for example in thin film transistor (TFT)-TN-LCDs, may result in the destruction of the thin film transistors underneath. For these reasons, the yield of optically satisfactory displays in LCD production has not been optimal to date.
A further disadvantage of rubbing is that it is not possible to produce structured orientation layers in a simple manner since the orientation direction cannot be varied locally during rubbing. Thus, mainly layers uniformly aligned over a large area can be produced by rubbing. However, structured orientation layers are of considerable interest in many areas of display technology and integrated optics. For example, the dependency of the angle of view of twisted nematic (TN) LCDs can thus be improved.
Orientation layers in which the orientation direction can be predetermined by exposure to polarized light have been known for some time. The problems inherent in rubbing can thus be overcome. In addition, it is possible to specify the orientation direction differently from region to region and hence to structure the orientation layer.
2. Description of the Prior Art
One possibility for the structured orientation of liquid crystals utilizes the isomerizability of certain dye molecules for inducing a preferred direction photochemically by exposure to polarized light of suitable wavelength. This is achieved, for example, by mixing a dye with an orientation polymer and then exposing said dye to polarized light. Such a guest/host system is described, for example, in U.S. Pat. No. 4,974,941. In this system, azobenzenes are mixed into polyimide orientation layers and then exposed to polarized light. Liquid crystals which are in contact with the surface of a layer exposed in this manner are oriented according to this preferred direction. This orientation process is reversible, i.e. the already established direction of orientation can be rotated again by further exposure of the layer to light having a second polarization direction. Since this reorientation process can be repeated as often as desired, orientation layers of this type are less suitable for use in LCDs.
A further possibility for producing highly resolved orientation patterns in liquid crystalline layers is described in Jpn. J. Appl. Phys. Vol. 31 (1992), 2155. In this process, the dimerization of polymer-bound photoreactive cinnamic acid groups, induced by exposure to linearly polarized light, is utilized for the structured orientation of liquid crystals. In contrast to the reversible orientation process described above, an anisotropic polymer network is established in the case of the photostructurable orientation layers described in Jpn. J. Appl. Phys. Vol. 31 (1992), 2155. These photo-oriented polymer networks can be used wherever structured or unstructured liquid crystal orientation layers are required. Apart from in LCDs, such orientation layers can also be used, for example, for the production of so-called hybrid layers, as exemplified in European Patent Applications EP-A-0 611 981, EP-A-0 689 084, EP-A-0 689 065 and EP-A-0 753 785. With these hybrid layers of photostructured orientation polymers and crosslinkable low molecular weight liquid crystals, it is possible to realize optical elements, such as, for example, nonabsorptive color filters, linear and circular polarizers, optical retardation layers, etc.
EP-A-611,786 describes cinnamic acid polymers which are suitable in principle for the production of such anisotropically crosslinked, photostructured orientation layers for liquid crystals. These crosslinkable cinnamic acid derivatives are in principle linked to the polymer main chain via the carboxyl function of the cinnamic acid (phenylacrylic acid) and a spacer. However, the photopolymers of this type which have been used to date have a number of serious disadvantages. Thus, for example, photochemical competing reactions adversely affect the orientability. In addition, the known cinnamic acid polymers have insufficient photochemical long-term stability. For example, prolonged exposure of a prefabricated orientation layer to UV light leads to the destruction of the orientation originally present. Multiple exposures in which an existing orientation layer having a predetermined recorded pattern is exposed again in order to orient the still unexposed parts in another direction can be carried out only if the previously exposed parts are covered by a mask. Otherwise, the already oriented parts of the layer may lose some or all of their structure as a result of photochemical secondary reactions.
A further disadvantage of the cinnamic acid polymers used to date is that there is no tilt angle in the case of the orientation surfaces comprising these materials, which surfaces are produced by simple exposure to polarized light. Particularly for use in LCDs, however, a tilt angle must also be provided by the orientation layer in addition to the orientation direction.
In the above-mentioned uniaxially rubbed polymer orientation layers, this tilt angle is already generated in the rubbing process on the polymer surface. If a liquid crystal is brought into contact with such a surface, the liquid crystal molecules are not parallel but inclined to the surface and the tilt angle is thus transmitted to the liquid crystal. The magnitude of the tilt angle is determined both by rubbing parameters, such as, for example, feed rate and pressure, and by the chemical structure of the polymer. For the production of liquid crystal displays, tilt angles between 1xc2x0 and 15xc2x0 are required, depending on the type. The larger tilt angles are required in particular for supertwisted nematic (STN) LCDs, in order to avoid the formation of so-called fingerprint textures. In TN and TFT-TN-LCDs, the direction of rotation and the tilting direction are defined by the tilt angle, with the result that xe2x80x9creverse twistxe2x80x9d and xe2x80x9creverse tiltxe2x80x9d phenomena are prevented. While reverse twist in the unswitched state results in regions with an incorrect direction of rotation, which is manifested visually in a mottled appearance of the display, reverse tilt is optically very troublesome, especially on switching the LCD by tilting the liquid crystals in different directions. Reverse twist can be prevented by doping the liquid crystal mixture with a chiral dopant of suitable direction of rotation. For suppressing reverse tilt, however, there is to date no alternative possibility to the use of orientation layers with a tilt angle.
It was therefore the object of the invention to produce photoreactive polymers which do not have the above disadvantages of the cinnamic acid polymers used to date, i.e. the lack of photochemical long-term stability and especially the lack of a tilt angle after exposure to polarized light, and are thus capable of producing stable, highly resolved orientation patterns.
Surprisingly, it has now been found that the polymers which are disclosed in EP-A-611 786 and are linked by a spacer to the carboxyl group (COO) or the carboxylamino group (xe2x80x94CONRxe2x80x94) of 3-arylacrylic acid derivatives as the photoreactive unit, it being possible for the arylacrylic acid derivatives to have 1-3 rings, lead to orientation layers having a substantial tilt angle if at least one ring is a phenylene radical which is substituted ortho or meta to one of the linkage points by at least one alkoxy or fluoroalkoxy group.
At the same time, the orientation layers are photochemically more stable and lead to excellent orientation of the liquid crystals, which manifests itself, for example, in very good contrast. Moreover, the exposure to linearly polarized light can generally be carried out in the advantageous longer-wavelength range or in advantageously shorter exposure times.
The present invention relates to polymers of the general formula I: 
in which
M1 and M1xe2x80x2 denote a recurring monomer unit from the group:
acrylate, methacrylate, 2-chloroacrylate, 2-phenylacrylate; acrylamide, methacrylamide, 2-chloroacrylamide and 2-phenylacrylamide which are optionally N-substituted by lower alkyl; vinyl ethers, vinyl esters, styrene derivatives, siloxanes;
M2 denotes a recurring monomer unit from the group:
acrylate, methacrylate, 2-chloroacrylate, 2-phenylacrylate, acrylamide, methacrylamide, 2-chloroacrylamide and 2-phenylacrylamide which are optionally N-substituted by lower alkyl; vinyl ethers, vinyl esters; straight-chain or branched alkyl esters of acrylic or methacrylic acid, allyl esters of acrylic or methacrylic acid, alkyl vinyl ethers or esters, phenoxyalkyl acrylates or phenoxyalkyl methacrylates or hydroxyalkyl acrylates or hydroxyalkyl methacrylates, phenylalkyl acrylates or phenylalkyl methacrylates, the alkyl radicals having 1 to 20, preferably 5 to 20, but in particular 5 to 18, carbon atoms; acrylonitrile, methacrylonitrile, styrene, 4-methylstyrene or siloxanes;
w, w1 and w2 are molar fractions of the comonomers with 0 less than wxe2x89xa61, 0xe2x89xa6w1 less than 1 and 0xe2x89xa6w2xe2x89xa60.5;
S1 and S1xe2x80x2, independently of one another, denote a spacer unit, such as a straight-chain or branched alkylene group xe2x80x94(CH2)rxe2x80x94 which is optionally monosubstituted or polysubstituted by fluorine, chlorine or cyano, or a chain of the formula xe2x80x94(CH2)rxe2x80x94Lxe2x80x94(CH2)sxe2x80x94, in which L denotes a single bond or linking functional groups, such as xe2x80x94Oxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OOCxe2x80x94, xe2x80x94NR1xe2x80x94, xe2x80x94NR1xe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94NR1xe2x80x94, xe2x80x94NR1xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94NR1xe2x80x94, xe2x80x94NR1xe2x80x94COxe2x80x94NR1xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94; in which R1 denotes hydrogen or lower alkyl and r and s each represent an integer from 1 to 20, with the proviso that r+s is xe2x89xa624;
D, Dxe2x80x2, independently of one another, denote xe2x80x94Oxe2x80x94 or xe2x80x94NR2xe2x80x94; in which R2 denotes hydrogen or lower alkyl;
X, Xxe2x80x2, Y and Yxe2x80x2, independently of one another, denote hydrogen, fluorine, chlorine, cyano, alkyl having 1 to 12 carbon atoms which is optionally substituted by fluorine and in which a CH2 group or a plurality of non-neighboring CH2 groups may optionally be replaced by O, xe2x80x94COOxe2x80x94, xe2x80x94OOCxe2x80x94 and/or xe2x80x94CHxe2x95x90CHxe2x80x94;
A and Axe2x80x2, independently of one another, denote phenylene, pyridine-2,5-diyl, pyrimidine-2,5-diyl, 1,3-dioxane-2,5-diyl, cyclohexane-1,4-diyl, piperidine-1,4-diyl or piperazine-1,4diyl which is unsubstituted or optionally substituted by fluorine, chlorine, cyano, alkyl, alkoxy or fluoroalkoxy;
B and Bxe2x80x2, independently of one another, denote phenylene, pyridine-2,5-diyl, pyrimidine-2,5-diyl, 1,4- or 2,6-naphthylene, 1,3-dioxane-2,5-diyl or cyclohexane-1,4-diyl which is unsubstituted or optionally substituted by fluorine, chlorine, cyano, alkyl, alkoxy or fluoroalkoxy;
C and Cxe2x80x2, independently of one another, denote phenylene which is unsubstituted or optionally substituted by fluorine, chlorine, cyano, alkyl, alkoxy or fluoroalkoxy, or pyrimidine-2,5-diyl, pyridine-2,5-diyl, 2,5-thiophenylene, 2,5-furanylene or 1,4- or 2,6-naphthylene;
K and Kxe2x80x2, independently of one another, denote hydrogen, fluorine, chlorine, cyano, nitro or a straight-chain or branched alkyl, alkoxy, alkyl-COO, alkyl-COxe2x80x94NR3 or alkyl-OCO group having 1 to 20 carbon atoms which is optionally substituted by fluorine, chlorine, cyano or nitro and in which one CH2 group or a plurality of non-neighboring CH2 groups may optionally be replaced by xe2x80x94Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94 and in which R3 denotes hydrogen or lower alkyl;
with the proviso that at least one of the rings A, B and C and/or at least one of the rings Axe2x80x2, Bxe2x80x2 and Cxe2x80x2 represents a phenylene radical which is substituted by at least one alkoxy group or fluoroalkoxy group, and, if K denotes alkoxy or fluoroalkoxy, at least one of the rings A, B and C and/or at least one of the rings Axe2x80x2, Bxe2x80x2 and Cxe2x80x2 represents a phenylene radical which is substituted by at least one further alkoxy group or fluoroalkoxy group;
Z, Zxe2x80x2, Z1 and Z1xe2x80x2, independently of one another, denote a single covalent bond, xe2x80x94(CH2)txe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94OCxe2x80x94, xe2x80x94NR4xe2x80x94, xe2x80x94COxe2x80x94NR4xe2x80x94, xe2x80x94R4Nxe2x80x94COxe2x80x94, xe2x80x94(CH2)uxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94(CH2)uxe2x80x94, xe2x80x94(CH2)uxe2x80x94NR4xe2x80x94 or xe2x80x94NR4xe2x80x94(CH2)uxe2x80x94; in which R4 denotes hydrogen or lower alkyl;
t denotes an integer from 1 to 4;
u denotes an integer from 1 to 3; and
p, pxe2x80x2, n and nxe2x80x2, independently of one another, denote 0 or 1.
The present invention also relates to the use of the polymers according to the invention as an orientation layer for liquid crystals, and to their use in optical components, in particular for the production of hybrid layer elements.
The polymers according to the invention can be used individually or in mixtures for the formation of orientation layers.
The polymer materials according to the invention are:
a) homopolymers having recurring structural units of the formula I, in which w is 1, w1 is 0 and w2 is 0.
b) copolymers having recurring structural units of the formula I, in which 0 less than w less than 1, 0 less than w1 less than 1, w2=0.
c) copolymers having recurring structural units of the formula I, in which 0 less than w less than 1, w1=0, 0 less than w2xe2x89xa60.5.
The copolymers stated under c) are preferred.
The homopolymers stated under a) are particularly preferred.
The polymers according to the invention have a molecular weight Mw between 1000 and 5,000,000, but preferably between 5000 and 2,000,000, but particularly advantageously between 10,000 and 1,000,000.
In the copolymers according to the invention and having recurring structural units of the formula I in which w1 is 0, the proportion of copolymers units of the radical M2 defined under formula I is less than or equal to 50%, preferably less than or equal to 30%, but in particular less than or equal to 15%.
The term xe2x80x9ccopolymersxe2x80x9d is understood as meaning preferably random copolymers.
Recurring monomer units (M1) and (M1xe2x80x2) are, for example, acrylates such as 
acrylamides, such as 
vinyl ethers and vinyl esters such as 
styrene derivatives such as 
siloxanes such as 
in which R1 denotes hydrogen or lower alkyl.
Preferred xe2x80x9cmonomer unitsxe2x80x9d M1 are acrylate, methacrylate, 2-chloroacrylate, acrylamide, methacrylamide, 2-chloroacrylamide, styrene derivatives and siloxanes.
Particularly preferred xe2x80x9cmonomer unitsxe2x80x9d M1 are acrylate, methacrylate, styrene derivatives and siloxanes.
Very particularly preferred xe2x80x9cmonomer unitsxe2x80x9d M1 are acrylate, methacrylate and styrene derivatives.
Recurring monomer units (M2) may likewise be the units mentioned under (M1) and additionally units such as, for example, straight-chain or branched alkyl esters, phenylalkyl esters, phenoxyalkyl esters, hydroxyalkyl esters or allyl esters of acrylic or methacrylic acid, such as 
in which R2 denotes alkyl, allyl or alkyl substituted by phenyl, phenoxy or hydroxyl.
Alkyl vinyl ethers or alkyl vinyl esters 
in which R3 denotes alkyl.
The term xe2x80x9clower alkylxe2x80x9d alone or in combination, such as xe2x80x9clower alkoxyxe2x80x9d, xe2x80x9chydroxy-lower alkylxe2x80x9d or xe2x80x9cphenoxy-lower alkylxe2x80x9d, designates straight-chain and branched saturated hydrocarbon radicals having 1 to 6, preferably having 1 to 3, carbon atoms, such as methyl, ethyl, propyl or isopropyl and the like.
The term xe2x80x9calkylxe2x80x9d alone or in combination, such as xe2x80x9calkoxyxe2x80x9d, designates straight-chain or branched saturated hydrocarbon radicals having up to 20 carbon atoms.
The term xe2x80x9cfluoroalkoxyxe2x80x9d designates an alkoxy radical as defined above, in which the hydrocarbon radicals are mono- or polysubstituted by fluorine. Examples of fluoroalkoxy groups are: 1-fluoropropoxy, 1-fluoropentyloxy, 2-fluoropropoxy, 2,2-difluoropropoxy, 3-fluoropropoxy, 3,3-difluoropropoxy and 3,3,3-trifluoropropoxy.
In the context of the present invention, preferred xe2x80x9cspacer unitsxe2x80x9d are a straight-chain or branched alkylene group xe2x80x94(CH2)rxe2x80x94, as well as xe2x80x94(CH2)rxe2x80x94Oxe2x80x94(CH2)sxe2x80x94, xe2x80x94(CH2)rxe2x80x94COOxe2x80x94(CH2)sxe2x80x94, xe2x80x94(CH2)rxe2x80x94OOCxe2x80x94(CH2)sxe2x80x94, xe2x80x94(CH2)rxe2x80x94NR1xe2x80x94COxe2x80x94(CH2)sxe2x80x94 or xe2x80x94(CH2)rxe2x80x94NR1COOxe2x80x94(CH2)sxe2x80x94, in which R1 denotes hydrogen or lower alkyl and r and s are each an integer from 1 to 20, but in particular 2 to 12, with the proviso that r+sxe2x89xa624.
Examples of preferred xe2x80x9cspacer unitsxe2x80x9d are 1,4-butylene, 1,5-pentylene, 1,6-hexylene, 1,7-heptylene, 1,8-octylene, 1,9-nonylene, 1,10-decylene, 1,11-undecylene, 1,12-dodecylene, 1,3-butylene, 3-methyl-1,3-butylene, 3-propyleneoxy-6-hexylene, 3-propylenecarbamoyloxy-6-hexylene, 3-propylenecarbonyloxy-6-hexylene, 3-propyleneoxycarbonyl-6-hexylene, 3-propylenecarbonylamino-6-hexylene, propylenecarbamoylhexylene and the like.
Particularly preferred xe2x80x9cspacer unitsxe2x80x9d are a straight-chain alkylene group xe2x80x94(CH2)rxe2x80x94, as well as xe2x80x94(CH2)rxe2x80x94Oxe2x80x94(CH2)sxe2x80x94, xe2x80x94(CH2)rxe2x80x94NHxe2x80x94COxe2x80x94(CH2)sxe2x80x94 or xe2x80x94(CH2)rxe2x80x94NHxe2x80x94COOxe2x80x94(CH2)sxe2x80x94, in which r and s are each an integer from 2 to 12 and the sum r+s is xe2x89xa624, in particular xe2x89xa615.
The term xe2x80x9cphenylenexe2x80x9d includes 1,2-, 1,3- and 1,4-phenylene. 1,3- and 1,4-Phenylene are preferred, in particular 1,4-phenylene.
In the compounds of the formula I, it is essential that at least one phenylene radical is present which carries at least one alkoxy or fluoroalkoxy group. If the group K is an alkoxy group or fluoroalkoxy group, at least one further alkoxy or fluoroalkoxy substituent must be present and must be located either on the same phenylene radical as the radical K or on another phenylene radical contained in the compound of the formula I.
As explained above, copolymers having recurring structural units of the formula I in which w1 is 0 are preferred and homopolymers having recurring structural units of the formula I in which w1 is 0 and w2 is 0 are particularly preferred. The preferred and particularly preferred compounds of the formula I can furthermore be divided into
a) compounds of the formula I having three rings A, B and C
b) compounds of the formula I having two rings B and C
c) compounds of the formula I having one ring C.
Among these in turn, compounds of the formula I which have one ring or two rings are preferred. With regard to the crosslinkable group on the aromatic ring C, compounds of the formula I in which X and Y denote hydrogen and D denotes oxygen are preferred.
Thus, compounds of the formula I in which M1, M2, S1 and K have the meaning stated under formula I and in which
D denotes oxygen;
X and Y denote hydrogen;
w denotes 0 less than w less than 1;
w1 denotes 0;
w2 denotes 0 less than w2 less than 0.5;
p denotes 0;
n denotes 0 or 1;
Z1 denotes a single covalent bond, xe2x80x94CH2CH2xe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94CH2xe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94CH2xe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94 or xe2x80x94Oxe2x80x94OCxe2x80x94;
C denotes phenylene, pyridine-2,5-diyl or pyrimidine-2,5-diyl which is unsubstituted or substituted by alkoxy or fluoroalkoxy; and
B denotes phenylene, pyridine-2,5-diyl, pyrimidine-2,5-diyl, cyclohexane-1,4-diyl or dioxane-1,5-diyl which is unsubstituted or substituted by alkoxy or fluoroalkoxy; with the proviso that at least one of the rings B and C represents a phenylene radical which is substituted by at least one alkoxy group or fluoroalkoxy group and, if K denotes alkoxy or fluoroalkoxy, at least one of the rings B and C represents a phenylene radical which is substituted by at least one further alkoxy group or fluoroalkoxy group.
Homopolymeric compounds of the formula I in which M1, S1, D, X, Y, K, Z1, B and C have the above-mentioned meaning and in which p is 0, n is 0 or 1, w is 1 and w1 and w2 are 0 are also preferred.
Particularly preferred are compounds of the formula I in which M1, M2, S1 and K have the meaning stated under formula I and in which
D denotes oxygen;
X and Y denote hydrogen;
w denotes 0 less than w less than 1;
w1 denotes 0;
w2 denotes 0 less than w2 less than 0.5;
p denotes 0;
n denotes 0 or 1;
B denotes phenylene which is unsubstituted or substituted by alkoxy or fluoroalkoxy, or cyclohexane-1,4-diyl; and
C denotes phenylene which is unsubstituted or substituted by alkoxy or fluoroalkoxy, with the proviso that one of the phenylene radicals present is substituted by at least one alkoxy group or fluoroalkoxy group and, if K denotes alkoxy or fluoroalkoxy, at least one of the phenylene radicals present is substituted by at least one further alkoxy group or fluoroalkoxy group.
Homopolymeric compounds of the formula I in which M1, S1, D, X, Y, K, B and C have the above-mentioned meaning and p is 0, n is 0 or 1, w is 1 and w1 and w2 are 0 are particularly preferred.
Polymers of the formula I are distinguished by the fact that they are easily obtainable. The preparation methods are known per se to the skilled worker.
The polymers of the formula I can in principle be prepared by two different processes. Apart from the direct polymerization of monomers prepared beforehand, it is possible to subject the reactive cinnamic acid derivatives to a polymer-analogous reaction with functional polymers.
For the direct polymerization, the monomers are first prepared from the individual components, i.e. the precursors of the compounds for the formula I, the spacers S1 and the polymerizable moieties M. The formation of the polymers is then carried out in a manner known per se. The polymerization can be carried out, for example, in the melt or in solution in the absence of oxygen and in the presence of a free radical initiator which is capable of generating free radicals thermally, photochemically or by redox reaction. The reaction can be carried out in a temperature range from xe2x88x9210xc2x0 C. to 120xc2x0 C., preferably in a range from 20xc2x0 C. to 100xc2x0 C.
For the production of the orientation layers, the polymers according to the invention must first be applied to a carrier. Examples of known carrier materials are aluminum oxide, titanium dioxide, silicon dioxide (glass or quartz) or mixed oxides, such as, for example, indium tin oxide (ITO). In the applications according to the invention for optical or electro-optical devices, glass or optionally a carrier coated with an electrode (for example a glass sheet coated with indium tin oxide (ITO)) are particularly important as carrier materials. For the application, the polymers are applied to a carrier in a spin-coating apparatus so that homogeneous layers of 0.05-50 xcexcm thickness are formed. The layers can be dimerized by exposure to linearly polarized light. By spatially selective irradiation of the molecular units of the formula I coupled to the carrier, very specific regions of a surface can now be oriented and at the same time also stabilized by the dimerization.
Thus, for the production of orientation layers in selected areas, the regions to be oriented can be exposed, for example, to a high-pressure mercury lamp, a xenon lamp or a pulsed UV laser with the use of a polarizer and optionally a mask for reproducing structures. The exposure time is dependent on the power of the individual lamps and may vary from a few seconds to one hour. However, the dimerization can also be effected by irradiation of the homogeneous layer with the use of filters which, for example, let through only the radiation suitable for the crosslinking reaction.
The polymers according to the invention are further illustrated by the Examples below. In the Examples below, g denotes the number of recurring units, so that polymers having a molecular weight Mw between 1000 and 5,000,000, preferably between 5000 and 2,000,000, but particularly advantageously between 10,000 and 1,000,000, result; w, w1 and w2 denote molar fractions of the comonomers with 0 less than wxe2x89xa61, 0xe2x89xa6w1 less than 1 and 0xe2x89xa6w2xe2x89xa60.5.